Air tools are often used to apply torque to a work piece. For example, a nutrunner air tool may be used to provide relative rotation between a nut and bolt to run the nut along the bolt to make a threaded connection. The torque applied substantially increases under load when the threaded connection approaches being fully installed. Torque shut-off valves have been developed for use in such air tools automatically to shut off the air supply to the motor when a preselected torque level is achieved.
For example, Whitehouse U.S. Pat. No. 3,373,824 discloses a spool type torque shut-off valve extending across the tool body with its ends protruding therefrom. This spool valve was normally held open by an adjustable coil spring acting against a first side of the spool valve. The spool valve could be closed when the motor back pressure operating on the second opposed side of the spool valve exceeded the selected spring pressure when preselected motor torque levels were reached. The torque shut-off valve in U.S. Pat. No. 3,373,824 would not close if the selected spring pressure was not exceeded. This would result in a low torque condition. This failure to close could result, for example, when additional tools were being operated on a common air system reducing the line pressure of the air supplied to the tool. In addition, the air tool with torque shut-off shown in U.S. Pat. No. 3,373,824 was ergonomically disadvantageous because of the valve ends projecting out of both sides of the tool, created wiring problems on instrumented models because the valve extended normal to the longitudinal axis of the tool and was subject to valve wear because the main air flow through the shut-off valve created side load forces on the spool valve body.
To overcome the failure to close when low torque had been achieved in reduced line pressure situations, torque shut-off valves were developed for air tools wherein controlled line pressure on one side of the spool valve acted against air pressure at the motor on the other side of the spool valve. Two variations of comparing controlled air line pressure to motor air pressure as shown in Whitehouse U.S. Pat. No. 4,434,858 (comparing controlled line pressure to motor back pressure) and Willoughby U.S. Pat. No. 4,243,111 (comparing controlled air line pressure to motor exhaust pressure). The shut-off valves in U.S. Pat. Nos. 4,434,858 and 4,243,111 solved the problem of the failure to close when the motor back pressure did not exceed the preselected spring pressure but failed to solve the other problems present in the tool disclosed in U.S. Pat. No. 3,373,828. Specifically, the Whitehouse '858 and Willoughby patents disclose shut-off valves positioned across the tool and extending outwardly from the tool profile to create ergonometric and wiring problems. These shut-off valves are also prone to potential wear problems because the main air passage side loads the slidable spool valve. Finally, these valves have external vents for the bias air, and oil can pass through the external vents and accumulate on the tool handle.